An intake controller is known in which timing for opening intake valves is substantially fixed, and the effective intake stroke is variably regulated by altering the timing for closing the intake valves based on an engine load in an attempt to reduce the pumping loss. Such an intake controller is disclosed in published Japanese Patent Application No. 55-87835.
However, in the intake controller disclosed published Japanese Patent Application No. 55-87835, because the timing for closing the intake valves comes significantly ahead of bottom dead center BDC in the intake stroke, the intake air in the cylinders expands adiabatically to bottom dead center BDC despite the suction stroke, so that the temperature inside the cylinders also drops as the pressure inside the cylinders drops. Although the compression stroke begins once bottom dead center BDC is passed, because almost nothing but adiabatic expansion and compression are present until the in-cylinder pressure at which the adiabatic expansion began is reached, that is, the in-cylinder pressure has merely been restored; the actual compression begins from the point at which the in-cylinder pressure is restored. Thus, the actual compression ratio drops significantly as the timing for closing the intake valves comes further ahead of bottom dead center BDC. Because this drop in the actual compression ratio results in a significant drop in the temperature of the air-fuel mixture inside the cylinders at compression top dead center TDC, the combustion rate drops due to the deteriorated combustion state. According, fuel economy is adversely impacted.
Also, when the timing for closing the intake valves is advanced in an engine equipped with a variable valve operating mechanism, it is feasible to improve the thermal efficiency of the engine by adhering a heat insulating material on the wall surfaces of the combustion chambers, partially or entirely, to prevent the pumping loss reduction effect from being lost. However, because the heat transfer rate increases when heat insulating materials, such as ceramic material, are used, at a high temperature generated at high engine load, the temperature of the intake air inside the combustion chambers increases so that undesirable knocking occurs. In addition, intake air filling efficiency at high load is also essential in that the volume of air is decreased to the extent that the temperature of the intake air increases, and a tradeoff occurs, resulting in a drop in torque